Abstract

A series-elastic actuator (SEA) can provide remarkable performance benefits in a robotic system, allowing the execution of highly dynamic manuevers, such as a jump. While SEAs have been used in numerous robotic systems, no comprehensive understanding of an optimal design exists. This paper presents a new analytical basis for maximizing an SEA thrust performance for jumping from rest with an articulated leg. The analytical SEA model is validated with simulation and experimental results from a prototype leg. An SEA decouples the dynamic limitations of a dc motor actuator from the joint, allowing larger lift-off velocities than with a directly driven joint. A detailed analysis of the complex dynamic response of an SEA during the thrust phase leads to a new maximum impulse criterion, where motor speed is approximately half the no-load speed at the moment of peak motor torque. The analytical model and this proposed criterion are used to develop a simple equation for selecting SEA design parameters. Lastly, a novel unidirectional SEA design is presented that allows for accurate positioning of the leg during flight.

Figures

A contour plot of simulated jump heights across a range of spring constants and gear ratios. The predicted Ks from Eq. 26 (solid line) yields a spring and gear ratio combination that produces a jump within 5% of the maximum jump height for either parameter.

The geometric foot position error from the calibration point is shown for the last 100ms of flight before touchdown. The USEA case is represented with a thick line, and the case with the USEA removed is represented with a thin line.

Comparison of body lift-off velocities ẏb for a DDA at both joints and an SEA at the knee, as computed from Eqs. 16,18, respectively. Typical spring torques at lift-off measured in the prototype leg are around 20Nm.

Comparison of body lift-off velocities ẏb for a DDA at both joints and an SEA at the knee, determined from the dynamic simulations. Both the series-elastic simulator and the experimental data obtain higher lift-off velocities than a DDA.

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